1. Field of the Invention
The present invention relates to a method and apparatus for ion attachment mass spectrometry used for a quantitative analysis for accurately measuring the concentration of a gas to be detected.
2. Description of the Related Art
An ion attachment mass spectrometer is a mass spectrometer designed to accurately measure the molecular weight of the gas to be detected. The analysis executed by this apparatus enables ionization and mass spectrometry of the detection gas without causing cracking. The ion attachment mass spectrometer has been reported in Hodge, Analytical Chemistry, vol. 48, no. 6, p. 825 (1976); Bombick, Analytical Chemistry, vol. 56, no. 3, p. 396 (1984); and Fujii et al., Analytical Chemistry, vol. 1, no. 9, p. 1026 (1989), Chemical Physics Letters, vol. 191, no. 1.2, p. 162 (1992), and Japanese Unexamined Patent Publication (Kokai) No. 6-11485.
The conventional ion attachment mass spectrometers will be explained referring to the drawings.
FIG. 9 shows the apparatus proposed by Fujii. In FIG. 9, 901 indicates a reaction chamber, 902 a first differential evacuation chamber, 903 a second differential evacuation chamber, 904 an analysis chamber, 905 a gas introduction mechanism, 906 an evacuation mechanism, and 907 a data processor. Further, 911 indicates an emitter, 912 a first aperture, 913 a reaction chamber seal, 914 a reaction chamber vacuum gauge, and 915 a baking mechanism. Further, 921 indicates a second aperture, 922 a partition of the first differential evacuation chamber, 931 a third aperture, 932 a partition of the second differential evacuation chamber, 933 an electrostatic lens, and 941 a Q-pole mass spectrometer. Further, 951 indicates a space to be measured, 952 a pipe, and 953 a flow rate adjustment valve. Reference numeral 961 indicates a first differential evacuation chamber wet pump, 962 a second differential evacuation chamber wet pump, and 963 an analysis chamber wet pump.
The reaction chamber 901, the first differential evacuation chamber 902, the second differential evacuation chamber 903, and the analysis chamber 904 form a vacuum chamber, that is, a chamber of a reduced pressure atmosphere being not more than atmospheric pressure. In the reaction chamber 901, an oxide of an alkali metal of an emitter is heated to cause the emission of Li+ and other positively charged metal ions. The detection gas is introduced into the reaction chamber 901. The metal ions gradually attach to (associate with) locations where the charges of the gas molecules concentrate and the molecules as a whole are ionized. The excess energy at the time of attachment is an extremely small 0.435 to 1.304 eV/molecule and there is less occurrence of disassociation.
Since the excess energy is low, however, if left as it is, the Li+ ends up being detached from the molecules again, so the total pressure of the reaction chamber 901 is made about 100 Pa to absorb the excess energy due to the large number of collisions. The gas absorbing the excess energy is neither the attaching ions or gas to be attached to, so is normally called a xe2x80x9cthird component gasxe2x80x9d.
The third component gas may also be the detection gas itself, but normally a low reactivity N2 gas etc. is used. Further, as the third component gas, there are sometimes cases of a base gas containing the detection gas from the start in the measurement space (carrier gas) or gas separately introduced by the reaction chamber 901. Due in part to contamination and other reasons, since the partial pressure of the detection gas introduced is normally not more than 1 Pa, almost all of the total pressure of the reaction chamber 901 of about 100 Pa becomes the partial pressure of the third component gas.
The gas molecules (ions) to which the metal ions are stably attached pass through the opening of the aperture and enter the first differential evacuation chamber 902. The first differential evacuation chamber 902 functions to connect in vacuum the reaction chamber 901 which should be set at about 100 Pa and the analysis chamber 904 which should be set at not more than 1xc3x9710xe2x88x923 Pa. This results in a total pressure of 0.1 to 10 Pa in the first differential evacuation chamber 902. The electrostatic lens 933 is provided in the second differential evacuation chamber 903. The ions are condensed and enter the analysis chamber 904. The Q-pole mass spectrometer 941 placed in the analysis chamber 904 breaks down and detects the entering ions for each mass of the gas molecules (ions) by electromagnetic force. The Q-pole mass spectrometer 941 outputs a mass signal showing the intensity for each mass number to the data processor 907. Note that the pressure inside the analysis chamber 904 has to be maintained at not more than 1xc3x9710xe2x88x923 Pa in order to operate the Q-pole mass spectrometer 941 normally.
FIG. 10 shows the apparatus proposed by Bombick, while FIG. 11 shows the apparatus proposed by Hodge. In FIG. 10 and FIG. 11, components substantially the same as those explained in FIG. 9 are given the same reference numerals. In the apparatus shown in FIG. 10, the reaction chamber 901 is arranged in the first differential evacuation chamber 902. The total pressure of the reaction chamber 901 is not measured. In the apparatus shown in FIG. 11 as well, the reaction chamber 901 is placed in the first differential evacuation chamber 902, but in this case the total pressure of the reaction chamber 901 is measured. Since a long pipe 970 is extended from the reaction chamber 901 and the vacuum gauge 914 attached, accurate measurement of the total pressure is difficult. The rest of the configuration is the same as explained above.
The ion attachment mass spectrometers have been developed as modifications of the chemical ionization mass spectrometers (CIMS) designed for measurement of the molecular weight of the detection gas. In the CIMS, a methane or another reaction gas is ionized by the electron impact to ionize the detection gas to positive charges or negative charges by an ion-molecule reaction. The mechanism of ionization is extremely complicated. Phenomena such as (1) hydrogen ion bonding of the reaction gas, (2) hydrogen ion draining from the detection gas, and (3) charge movement occur. The bonding energy in the case of hydrogen ion bonds is so large as to be 6.957 to 8.696 eV/ molecule, and therefore dissociation often ends up occurring. Peaks of the molecular ions are sometimes observed depending on the type of gas.
Originally, the CIMS was designed for measurement of the molecular weight of the detection gas, that is, xe2x80x9cqualitative analysisxe2x80x9d for obtaining information on xe2x80x9cwhat are the compositionsxe2x80x9d. Therefore, the ion attachment mass spectrometer is confirmed to be effective for the qualitative analysis of organic substances or radicals. The ion attachment mass spectrometer, however, suffers from problems such as the stability of the mass signal and therefore is not used at all in industry for the qualitative analysis.
Analysis going further from the qualitative analysis and obtaining information on xe2x80x9cwhat kind of composition is present in what amountxe2x80x9d is called xe2x80x9cquantitative analysisxe2x80x9d. Due to the following reasons, however, the ion attachment mass spectrometers have not been used for the quantitative analysis at all.
First, the quantitative analysis will be explained. In the quantitative analysis, four factors are important: (1) the applicable samples, (2) the signal-to-noise ratio, (3) the signal stability, and (4) the background (interference peaks). The xe2x80x9capplicable samplesxe2x80x9d is the extent of the types of the samples which can be applied, the xe2x80x9csignal-to-noise ratioxe2x80x9d is the ratio of the mass signal (peak height) and noise (amount of fast cycle fluctuation of base level), the xe2x80x9csignal stabilityxe2x80x9d is the reproducibility of the mass signal, and the xe2x80x9cbackground (interference peaks)xe2x80x9d is the peaks not inherently present which change the apparent mass signal (peak height).
At the present, electron impact mass spectrometers (EIMS) and atmosphere pressure ion mass spectrometers (APIMS) are being used for the quantitative analysis.
With the EIMS, the applicable samples are good, but there are problems in the signal-to-noise ratio or the background (interference peaks). That is, the vacuum ultraviolet light from the gas receiving the electron impact becomes a cause of noise, so even if the mass signal is increased by, for example, increasing the electron current, the amount of noise will also end up increasing and as a result the signal-to-noise ratio will not be improved much at all. Further, fragment peaks resulting from cracking due to the electron impact easily become the interference peaks.
On the other hand, with the APIMS, the signal-to-noise ratio is good, but there are problems in the signal stability or background (interference peaks). That is, since a corona discharge is used at an atmospheric pressure, it is difficult to secure the stability. The clusters occurring due to the ion-molecule reactions at the atmospheric pressure easily became the interference peaks.
As opposed to the EIMS and APIMS, with the CIMS, there were problems with all four of the above factors. Therefore, this spectrometer is not being used much at all for the quantitative analysis. Since the conventional IAMS, like the CIMS, it had problems in the four factors, it was not used for the quantitative analysis.
Next, an explanation will be made of the background (interference peaks) and the vacuum technology relating to it.
In an ideal evacuation process, it is known that the pressure is reduced by the exponential function exe2x88x921. The value (V/S) of the evacuated volume (V) divided by the pumping speed (S) is defined as the xe2x80x9cevacuation time constantxe2x80x9d. When a time corresponding to the evacuation time Constant elapses, the pressure falls to 37 percent of or exe2x88x921. After the elapse of five times that amount of time, the pressure falls to 1 percent or exe2x88x925. Therefore, the evacuation time constant at the reaction chamber 901 corresponds to the response of measurement and determines by what extent of delay the change in concentration of the detection gas in the reaction chamber 901 tracks the change in concentration in the detection space.
The pumping speed controlling the evacuation time constant becomes the substantive pumping speed determined by the pumping speed of the vacuum pump itself and the conductance of the pipes etc. in the middle, that is, the effective pumping speed. In the conventional ion attachment mass spectrometer, the vacuum pump was not directly attached to the reaction chamber. Evacuation was performed through an opening of the aperture member. With this type, the effective pumping speed for the reaction chamber is greatly influenced by the conductance of the opening. The conductance of the opening is proportional to the opening area, so when the opening area is small, the effective pumping speed becomes small and the evacuation time constant becomes larger. In the conventional ion attachment mass spectrometer, however, since the aperture member having a relatively large opening area was used, the spectrometer had a relatively fast response defined by the evacuation time constant being not more than 1 second.
With the conventional ion attachment mass spectrometer of the related art, however, there was another problem of the gas dwelling in the reaction chamber 901. Even after the evacuation time constant sufficiently passed, the gas was not completely replaced and therefore the phenomenon of previous hysteresis remaining (memory effect) occurred. The property of the apparatus controlling the memory effect can be evaluated in the following way as the xe2x80x9cdwell ratexe2x80x9d of the gas. The xe2x80x9cdwell ratexe2x80x9d is defined by the ratio of the previous gas remaining (residual ratio) at the time that a time equivalent to five times the evacuation time constant elapses when changing the gas introduced into the reaction chamber to a different gas instantaneously. To eliminate this effect of the evacuation time constant, however, the value of the actually measured residual ratio minus 1 percent (=exe2x88x925) is defined as the accurate dwell rate.
The total pressure of the reaction chamber 901 of the ion attachment mass spectrometer is normally made about 100 Pa, but with the total pressure, the flow of the gas becomes viscous, the gas molecules collide with each other, and the flow becomes like that of a gentle river overall. Therefore, if there are corners or depressions in the reaction chamber 901, pockets of flow will be created there and the dwell rate will end up increasing. In the conventional ion attachment mass spectrometer designed for the qualitative analysis, however, the dwell rate was not a problem, so there were many corners or depressions in the reaction chamber 901. Note that the ionization chamber for the EI has a pressure of 1xc3x9710xe2x88x923 Pa or so, so a molecular flow results. The gas molecules collide with only the walls and diffuse randomly, so there is now clear flow as a whole and even if there are the corners or depressions, dwelling does not occur.
Vacuum pumps can be roughly divided into xe2x80x9cwet pumpsxe2x80x9d using an oil working fluid and xe2x80x9cdry pumpsxe2x80x9d not using the oil working fluid. The xe2x80x9cworking fluidxe2x80x9d is a fluid for driving the evacuation operation. Normally, oil is used. The wet pumps include, for high pressure use, oil rotary pumps (RP) and, for low pressure use, oil diffusion pumps (DP). The dry pumps include, for high pressure use, membrane pumps, scroll pumps, screw groove pumps, and axial flow molecular pumps and, for low pressure use, turbo molecular pumps (TMP), ion pumps, and getter pumps. In the conventional ion attachment mass spectrometer designed for the qualitative analysis, a bit of oil contamination was not a problem, so the wet pump was used for evacuation of the reaction chamber in the all case.
Among the conventional ion attachment mass spectrometers, the apparatus of Fujii had a large opening, so its pressure reached was low, but it used the RP with a large evacuation flow rate, while the apparatuses of Hodge and Bombick had small openings, so they had small evacuation flow rates, but used the DPs with high pressures reached. Whatever the case, in the ion attachment mass spectrometers using the wet pumps for evacuation of the reaction chamber, the reaction chamber is contaminated by the oil, so, while slight, interference peaks due to the oil components are caused and major problems arise for the quantitative analysis. Even if there is no contamination from the pump, a gas is emitted from the inside walls of the vacuum chamber or inside parts and becomes residual impurities in the vacuum chamber. The simplest way to reduce the emission of gas is baking. If the entire vacuum chamber is heated to 100 to 200xc2x0 C. to sufficiently cause the emission of gas while evacuating the chamber, and then the chamber is returned to room temperature, the emission of gas is greatly reduced. In the apparatus of Fujii, there is a baking mechanism for the reaction chamber, but in the apparatuses of Hodge and Bombick, the reaction chamber is built into the first differential evacuation chamber, so there is no exclusive baking mechanism.
Further, vacuum seal materials are classified into polymer organic based materials such as rubber and Teflon and metal-based materials such as copper and aluminum. Polymer organic-based materials have the advantages of having a small clamping force and being able to adapt to complicated shapes, so have a high reliability and also are inexpensive in price, but have the disadvantages of the susceptibility to emission of gas from the materials or passage of gas from the high pressure side. Metal-based materials have features in sharp contrast to the above material. Therefore, in the conventional ion attachment mass spectrometers designed for the qualitative analysis, since gas emission or gas passage was not a problem, polymer organic-based seal materials were frequently used. In particular, since the reaction chamber becomes complicated in a shape, many polymer organic-based seal materials were used.
The inside wall surface of the vacuum chamber is sometimes treated by polishing, immobilization, and precision washing with the aim of reducing the gas emission. As polishing, acid pickling, electrolysis, buffing, shot blasting, electrolytic compounding, chemical, and other methods are known. As immobilization, the method of formation of a Cr oxide film, Si oxide film, or other film, the method of forming an oxide film of the material by heating in an oxidizing atmosphere, etc. are known. The precision washing is a method of precise washing using at least two types of solutions such as an alkali degreasing solution or purified water. These surface treatments have recently been put into practical use for semiconductor fabrication facilities. This polishing, immobilization, precision washing, and other surface treatment have not been applied to the conventional ion attachment mass spectrometers designed for qualitative analysis.
In this way, if backflow of oil from the evacuation mechanism or emission of gas from the reaction chamber occurs, the amount (partial pressure) of residual impurities present in the reaction chamber during measurement will increase and components which should not be present in the detection space will be measured. The degree of the effect can be evaluated as xe2x80x9ca residual impurity ratexe2x80x9d, that is, the ratio of the partial pressure of the residual impurities to the total pressure during the measurement. The pressure reached when gas is not being introduced into the reaction chamber corresponds to the partial pressure of the total of the oil backflow and gas emission, so to actually find the residual impurity rate, the pressure reached in the reaction chamber should be divided by the total pressure during the measurement.
In the conventional ion attachment mass spectrometers, as mentioned above, there were considered to be problems in the four factors, that is, the applicable samples, signal-to-noise ratio, signal stability, and background (interference peaks). Therefore, these were not used for quantitative analysis. The present inventors, however, engaged in detailed studies of the ion attachment mass spectrometer only used for the special qualitative analysis in the past from a new perspective and as a result found that there were no inherent problems in the applicable samples or signal-to-noise ratio.
That is, for the applicable samples, they confirmed that the sensitivity was sufficient even with halogenated compounds with large electron affinitiesxe2x80x94for which analysis was previously thought impossible (this is already filed as Japanese Patent Application No.11-356725). Regarding this mechanism, it is believed that this is because the ease of attachment of positive ions has no relation to the ease of attachment of electrons (that is, the magnitude of the electron affinity) and is determined by the bias of the electron distribution. Regarding the signal-to-noise ratio, they found that unlike the EIMS, even if the mass signal level is increased, the noise will not increase and that the signal-to-noise ratio can be improved by various structural improvements. Regarding this mechanism, they believe the reasons are that the temperature of the filament is an extremely low 600xc2x0 C. (1800xc2x0 C. in the EIMS) and the more vacuum ultraviolet rays are emitted, the less the gas is excited.
Therefore, for using an ion attachment mass spectrometer as a practical quantitative analysis system, the issue becomes the improvement of the remaining two factors, that is, the signal stability and the background (interference peaks). The specific figures to be achieved differ depending on the object of measurement, but for general quantitative analysis, the following can be envisioned. Regarding signal stability, a signal stability of at least 1 to 10 percent is probably necessary. Regarding background, it is necessary for the change in concentration of the detection gas in the reaction chamber to accurately track the changes in concentration of the detection gas in the detection space, but the time factors are the evacuation time constant and the dwell rate. There probably have to be not more than 1 second and not more than 1 percent, respectively. Next, there may not be any gas other than the detection gas and the known third component gas in the reaction chamber, but the different types of gas are caused by contamination by the pump and emission of gas from the container. In both cases, the rate of residual impurities in the measurement must be not more than 1 ppm. In the conventional ion attachment mass spectrometers of the related art, these could not be realized. The reasons are not clear. Achieving these is a subject of the present invention.
An object of the present invention is to provide a method and apparatus for ion attachment mass spectrometry enabling a quantitative analysis.
The present inventors engaged in detailed studies on the signal stability required first of all in the quantitative analysis and as a result pinpointed the major factors inhibiting the signal stability in an ion attachment mass spectrometer. This is that, considered from the perspective of the ion attachment mass spectrometry apparatus shown in FIG. 9, the sensitivity is strongly dependent on the total pressure of the reaction chamber and the first differential evacuation chamber. Here, the xe2x80x9csensitivityxe2x80x9d is a ratio of the mass signal to the amount of a specific component present and is a coefficient used for calculating the true amount of presence (quantitative value) from the mass signal measured in the quantitative analysis. Further, the xe2x80x9ctotal pressurexe2x80x9d is the total of the pressures (partial pressures) of all of the component gases contained. Normally, the total pressure of the reaction chamber and the differential evacuation chamber is substantially equal to the partial pressure of the third component gas.
While the sensitivity of an ion attachment mass spectrometry apparatus has dependency on the total pressure, in the EIMS or APIMS of the related art, the sensitivity is believed not to change according to the total pressure. The fact that the sensitivity changes depending on the component is well known in the EIMS or APIMS as well. There are already sensitivity tables for different components. These have become essential for quantitative measurement. These sensitivity tables, however, are based on set conditions of electron energy etc., but there are no conditions set on the total pressure. This is due to the understanding that the sensitivity does not change depending on the total pressure. In the EIMS, the sensitivity does not depend on the total pressure because other gases do not have an effect on the process of ionization by electron impact and because the operating total pressure is a low 10xe2x88x923 Pa. In the APIMS, there is a possibility of dependency on the total pressure, but changes in sensitivity do not appear since the apparatus is constantly operating at a certain total pressure (atmospheric pressure). In the CIMS, the sensitivity appears to be dependent on the total pressure, but the dependency on total pressure is not clear due to other factors of instability.
FIG. 2 shows a graph of the dependency of sensitivity on the total pressure in a reaction chamber of an ion attachment mass spectrometry apparatus. The apparatus is basically the same as the apparatus according to the first embodiment explained later. The gas to be detected is for example H2O or C4F8. This graph is obtained by reading the changes in the mass signal when fixing the amounts of H2O and C4F8 present in the reaction chamber (fixed partial pressures of 1 Pa) and changing the partial pressure of the third component gas N2 between 10 to 300 Pa. In both cases, the changes are close to a second order function of a top projection. The position and magnitude of the maximum value differ depending on the type of gas.
The mechanism of the dependency of sensitivity on total pressure is believed to be as follows: When the total pressure in the reaction chamber increases, the rate of absorption of the excess energy becomes higher and the gas molecules (ions) with the metal ions stably attached increase. If the total pressure further increases, however, the amount of gas molecules (ions) with the metal ions stably attached will become saturated. On the other hand, the mean free distance will become smaller, and the gas molecules (ions) passing through the opening of the aperture member will decrease. These phenomena overlap each other, so the change becomes close to a second order function with a projecting top. Further, the degree of the phenomena differs according to the type of gas, so a difference appears in the dependency according to the type of gas.
In the ion attachment mass spectrometry apparatus shown in FIG. 9, the total pressure of the first differential evacuation chamber is determined by three quantities: The total pressure of the reaction chamber, the conductance of the first aperture, and the pumping speed of the first differential evacuation chamber dry pump. In the actual measurement, however, since the conductance of the first aperture and the pumping speed of the second differential evacuation chamber dry pump are constant, the total pressure of the differential evacuation chamber is in a one-to-one correspondence with the total pressure of the reaction chamber. Therefore, the data of FIG. 2 includes not only the dependency on total pressure of the reaction chamber, but also the first differential evacuation chamber corresponding to it and is perfect for the actual measurement. More strictly speaking, however, there may be changes in sensitivity due to only the total pressure of the differential evacuation chamber.
FIG. 3 is a graph of the dependency of sensitivity on total pressure in the first differential evacuation chamber in the state with a constant total pressure of the reaction chamber. There is relatively little difference depending on the type of gas, but the reduction in sensitivity is a change close to an exponential function. This is believed to be due to the fact that the problem of absorption of excess energy is irrelevant in the first differential evacuation chamber. Only the passage of gas molecules (ions) is relevant.
In the above measurement, the flow rate of N2 is changed to change the total pressure, but the N2 itself is not consumed by a reaction etc., so the cause of the dependency is clearly not the flow rate, but the total pressure. Therefore, it was learned that, in both the reaction chamber and the first differential evacuation chamber, the sensitivity is dependent on the total pressure and that, further, the dependency differs depending on the type of gas.
The fact that the sensitivity is dependent on the total pressure means that even if the amount of detection gas present is the same, if the total pressure changes, the mass signal (peak height) will end up changing. In the past, this was not recognized. The total pressure differed with each measurement or the total pressure fluctuated right in the middle of measurement, so a mass signal with a good reproducibility could not be obtained. This was the reason why quantitative analysis was not possible with ion attachment mass spectrometry apparatuses.
Therefore, to enable quantitative analysis in an ion attachment mass spectrometry apparatus, securing signal stability becomes essential. In the present invention, by securing signal stability, quantitative analysis by an ion attachment mass spectrometry apparatus is made possible. Further, it was learned that the following should be done to secure signal stability in an ion attachment mass spectrometry apparatus.
The total pressure of the reaction chamber or the reaction chamber and the first differential evacuation chamber should be made accurately measurable and the total pressure should be accurately set at a certain constant value. The magnitude of the total pressure should be made a total pressure corresponding to the sensitivity used for the calculation of the quantitative value. The fluctuation of the total pressure should be kept in the range enabling error of the quantitative signal due to changes in sensitivity to be kept within an allowable range. More preferably, in the region with little change of sensitivity, that is, the reaction chamber, the total pressure should be set to 100 to 250 Pa, while in the first differential evacuation chamber, it should be set to not more than 1 Pa. By satisfying the above conditions, it is made possible to secure signal stability in the ion attachment mass spectrometry apparatus and thereby perform quantitative analysis by the ion attachment mass spectrometry apparatus.
Further, the present inventors studied the reduction in the background (interference peaks) required second for quantitative analysis. As a result, they pinpointed four factors of xe2x80x9cevacuation time constantxe2x80x9d, xe2x80x9cdwell ratexe2x80x9d, xe2x80x9ccontamination by pumpxe2x80x9d, and xe2x80x9cgas emitted from containerxe2x80x9d and showed that these could be solved by the following means: (1) for the evacuation time constant, reduction of the inside volume of the reaction chamber and increase of the effective evacuation, (2) for the dwell rate, elimination of corners or depressions in the reaction chamber to make the flow of the gas smooth and in one direction, (3) for the contamination by the pump, use of a dry pump free from backflow of oil for evacuation of the reaction chamber, and (4) for the gas emitted from the container, use of a metal-based seal for the seal of the reaction chamber and treatment of the inside wall surface of the reaction chamber by polishing, immobilization, and precision washing.
From the above viewpoint, the method and apparatus for ion attachment mass spectrometry according to the present invention are configured as follows:
The first method of ion attachment mass spectrometry according to the present invention is a method for causing positively charged metal ions to attach to a detection gas in a reduced pressure atmosphere to ionize the gas for measurement of mass spectrometry, comprising utilizing the property that the sensitivity of each component of the detection gas has a dependency on the total pressure of the reduced pressure atmosphere and that the dependency on the total pressure differs for each component and performing quantitative analysis while using the total pressure data of the reduced pressure atmosphere measured at the time of mass spectrometry for processing of the mass spectrometry data of each component.
Further, a second method of ion attachment mass spectrometry according to the present invention is a method for causing positively charged metal ions to attach to a detection gas in a reduced pressure atmosphere to ionize the gas for measurement of mass spectrometry, comprising utilizing the property that the sensitivity of each component of the detection gas has a dependency on the total pressure of the reduced pressure atmosphere and that the dependency on the total pressure differs for each component and performing quantitative analysis while using the total pressure data of the reduced pressure atmosphere measured at the time of mass spectrometry for setting the measurement conditions for the mass spectrometry of each component.
In the above methods of ion attachment mass spectrometry, a quantitative value is calculated for each component using the sensitivity corresponding to the total pressure during measurement. In the calculation, the quantitative value is obtained by dividing the signal obtained by the mass spectrometer by a coefficient relating to the sensitivity for each component.
In the above methods of ion attachment mass spectrometry, the total pressure during measurement is set within an allowable fluctuation of total pressure.
In the above methods, the allowable fluctuation of total pressure is calculated for each component using a rate of change of sensitivity corresponding to the total pressure during the measurement and a required quantitative error value.
The first apparatus for ion attachment mass spectrometry according to the present invention is apparatus for measurement of mass spectrometry after causing positively charged metal ions to attach to a detection gas to ionize it through a reaction chamber and analysis chamber providing a reduced pressure atmosphere, provided with the reaction chamber for causing positively charged metal ions to attach to the detection gas; a mass spectrometer for mass separation and detection of the detection gas to which the positively charged metal ions are attached; the analysis chamber in which the mass spectrometer is placed; an introduction mechanism for introducing a gas containing the detection gas into the reaction chamber; an evacuation mechanism for evacuating the gas containing the detection gas; a data processor for receiving and processing a mass signal from the mass spectrometer; and a vacuum gauge for measuring the total pressure of the reduced pressure atmosphere; a total pressure signal from the vacuum gauge measured during the measurement being input to the data processor; the data processor being provided with a processing means for performing quantitative analysis of each component utilizing the fact that the sensitivity of each component has a dependency on the total pressure of the reduced pressure atmosphere and that the dependency on total pressure differs for each component.
The second apparatus for ion attachment mass spectrometry according to the present invention is a mass spectrometry apparatus having the above configuration and exhibiting the above actions, wherein provision is made of a vacuum gauge for measuring the total pressure of the reduced pressure atmosphere; a total pressure signal from the vacuum gauge measured during the measurement is input to the introduction mechanism or the evacuation mechanism; the data processor performs quantitative analysis of each component.
In the ion attachment mass spectrometry apparatus having this configuration, preferably a differential evacuation chamber of a reduced pressure atmosphere for connecting the reaction chamber and the analysis chamber in a vacuum state is provided between the reaction chamber and the analysis chamber.
In the above apparatus for ion attachment mass spectrometry, the total pressure signal is input to the data processor, and the data processing means of the data processor calculates a quantitative value of each component using a sensitivity corresponding to the total pressure during measurement and a mass signal. The processing means calculates the quantitative value by dividing the mass signal by a coefficient relating to sensitivity.
In the above apparatus for ion attachment mass spectrometry, the total pressure signal is input to the introduction mechanism or the evacuation mechanism, and the introduction mechanism or the evacuation mechanism is controlled using the total pressure signal so that the total pressure of the reduced pressure atmosphere becomes within an allowable fluctuation of total pressure.
In the above apparatus for ion attachment mass spectrometry, the total pressure signal is input to the data processor, and the data processor uses the total pressure signal to monitor that the total pressure of the reduced pressure atmosphere is within an allowable fluctuation of total pressure.
In the above apparatus for ion attachment mass spectrometry, the allowable fluctuation of total pressure is calculated from a rate of change of sensitivity corresponding to the total pressure of the reduced pressure atmosphere during measurement and a required quantitative error value.
In the above configuration, it is possible to freely select any reduced pressure atmosphere for measuring and controlling the total pressure. For example, it may be provided by the reaction chamber and differential exchange chamber or may be the reaction chamber. Further, the reduced pressure atmosphere for measuring the total pressure may be provided by the differential evacuation chamber, while the reduced pressure atmosphere for controlling the total pressure may be provided by the reaction chamber.
In the above configuration, the dependency of the sensitivity on the total pressure of the reaction chamber may be approximated by a second order function. Further, the dependency of the sensitivity on the total pressure of the differential evacuation chamber may be approximated by an exponential function.
The total pressure of the reaction chamber is preferably set, maintained, and measured in a region with a small rate of change of the sensitivity. The total pressure of the reaction chamber is preferably set and maintained at 50 to 250 Pa. Further, the total pressure of the differential evacuation chamber is preferably set, maintained, and measured in a region with a small rate of change of the sensitivity. The total pressure of the differential evacuation chamber is preferably set and maintained at no more than 1 Pa.
The introduction mechanism or evacuation mechanism is characterized in that feedback control is performed on the total pressure of the reduced pressure atmosphere by the total pressure signal from the vacuum gauge.
Further, use is made of an introduction mechanism and an evacuation mechanism with a total of the maximum rate of fluctuation of the amount of introduction and the maximum rate of fluctuation of the pumping speed smaller than the allowable rate of fluctuation of the total pressure of the reduced pressure atmosphere.
As the evacuation mechanism, it is preferable to use a dry pump. Further, as this dry pump, a turbo molecular pump, axial flow molecular pump, or screw groove pump is preferably used.
The inside volume of the reaction chamber and the effective pumping speed of the evacuation mechanism are preferably determined so that the evacuation time constant of the reaction chamber becomes no more than 1 second. Further, the inside shape of the reaction chamber and the effective pumping speed of the evacuation mechanism are preferably determined so that the rate of dwell in the reaction chamber becomes no more than 1 percent. Further, the amount of emission of gas of the reaction chamber and the partial pressure of impurities during operation of the evacuation mechanism are preferably determined so that the rate of residual impurities of the reaction chamber becomes no more than 1 ppm.
In the ion attachment mass spectrometry apparatus, it is preferable to use a diaphragm type vacuum gauge as the above vacuum gauge, it is preferable to provide a baking mechanism in the reaction chamber, and it is preferable to use a metal-based material for the seal material of the reaction chamber.
As clear from the above explanation, according to the present invention, the ion attachment mass spectrometry apparatus obtains the sensitivity characteristic, dependent on the total pressure, for the type of gas for which quantitative analysis is to be performed and performs predetermined processing on the mass signal obtained from the mass spectrometer using a coefficient relating to the sensitivity, so the ion attachment mass spectrometry apparatus can solve the original problem of signal stability and the problem of background (interference peak) in quantitative analysis.